This invention relates generally to the field of operational amplifiers; and, in particular, to an amplifier having minimal zero crossover distortion and quiescent current requirements.
Well known in monolithic integrated circuit design, the design of bias circuitry internal to the chip is essential since it determines the internal voltage and current levels over all operating conditions of the integrated circuit as well as over all manufacturing process variations. The industry trend for electronic systems encompassing operational amplifiers is evolving toward lower operating voltages supplied from battery sources. Thus, amplifiers are used in applications requiring low voltage single supply operations in addition to traditionally desired operational amplifier properties such as high input impedance, low input offset voltage, low noise, high bandwidth, high speed and sufficient output drive capabilities.
The operational amplifier consists of at least two stages: an input amplifier stage and an output stage. The input amplifier stage has the task of deriving the difference between the two inputs. The primary purpose of the output stage is voltage amplification. The output stage optionally includes a current boosting scheme which increases the amplifier""s load capacity. Conventionally, amplifier output stages have used techniques involving combinations of transistors including npn, pnp and metal oxide semiconductor field effect transistors to satisfy many performance specifications, such as low crossover distortion, high gain factor, large output voltage swings including rail to rail performance, excellent phase and gain margins, low output impedance and symmetrical source and sink capabilities. A well-designed output stage should achieve these performance specifications while consuming low quiescent power and not limiting the frequency response of the amplifier.
During operation, an output stage consumes current from a power supply. A portion of this current, known as the quiescent current, is used to bias the internal circuitry of z; the output stage. The quiescent current is the current required to bias all transistors on when the input is shorted to ground. The purpose of the quiescent current is to provide sufficient base current for the amplifier""s output transistors and current to maintain the amplifier""s critical circuitry xe2x80x9con.xe2x80x9d Low quiescent current is desirable because it reduces power consumption when the amplifier is operating at a light load, or with no load at all. The active components of an all NPN Class AB amplifier include two NPN transistor, wherein the emitter of the first transistor is connected to the collector of the second transistor. The first NPN transistor amplifies the input signal during the positive swing of the signal, and the second NPN transistor amplifies the input signal in its negative swing. The active components may include diodes, resistors and transistors, with increased bias current for the two transistors to reduce crossover distortion.
Traditional Class AB output stages of an operational amplifier are capable of driving a specified minimum impedance load while possessing a low quiescent current. Traditional Class AB output stage xe2x80x9csinkxe2x80x9d or xe2x80x9csourcexe2x80x9d a significantly larger current in the presence of low impedance loads. Without a quiescent current large enough to bias all transistors critical to the output stage, these transistors will be xe2x80x9ccut offxe2x80x9d in an effort to drive a low impedance load. Turning this critical circuitry on again, ready for the next amplifier output transition increases crossover distortion.
Conventional AB class output stages drive a minimum impedance load having a given crossover distortion. Where lower impedance loads exist, higher quiescent current is required within the output stage. Going beyond output stage""s specified limits of load results in increasingly considerable crossover distortion.
In conclusion, there are existing designs that minimize crossover distortion of the output stages of an amplifier by allowing the active devices to conduct a small amount of current at the crossover point. Some arrangements use both NPN and PNP output transistors which requires more space and operates slower than an all NPN output transistor arrangement. An arrangement utilizing all PNP transistors also requires more space and operates slower than an all NPN output transistor arrangement. These techniques are limited by the quiescent current; hence, a need exists for an output stage that significantly minimizes crossover distortion using an all NPN output transistor arrangement with no quiescent current requirements.
The present invention relates to an output stage, having transistors of the same polarity type, which function to amplify the input signal at a wide range of frequencies, with low power consumption and low crossover distortion. By biasing the output transistors to remain on during both the positive and negative voltage swing of the input signal, low power consumption as well as low output crossover distortion is achieved. A high efficiency, low crossover distortion, current amplifier circuit for amplifying an input signal in accordance with this present invention includes an output driver, a transistor, a resistor, a current source and a translinear loop circuit. The output driver includes a sourcing circuit. The current source, connected to the output driver, provides bias current to the sourcing circuit. The value of the resistor determines whether the transistor remains on at all times and should not be set such that reverse bias occurs in the sourcing circuit. This resistor enhances the ratio of transistor emitter area between the transistor and a second transistor of the sourcing circuit. The translinear loop circuit, connected to the output driver, receives the input signal. The translinear loop includes a sinking circuit, such that the translinear loop circuit is responsive to the sinking current signal and operable to provide a bias current signal proportional to the sinking current.